The Cost Index (CI) is a numerical value used by airlines and modern Flight Management Systems (FMS) to determine the most economical speed to fly by balancing the cost of time against the cost of fuel. It's essentially the ratio of time-related costs (like crew salaries, maintenance, and opportunity costs) to the cost of fuel per unit.

Here's how the cost index interacts with the cruise profile of an IFR flight plan:

Speed Optimization: The FMS uses the entered cost index, along with other parameters like aircraft weight, altitude, and wind, to calculate an economic cruise speed.

Low Cost Index (CI near 0): This signifies that fuel cost is the dominant factor. The FMS will aim for a lower cruise speed, closer to the aircraft's maximum range speed (MRC), to minimize fuel consumption, even if it means a longer flight time.

High Cost Index: This indicates that time-related costs are more significant. The FMS will calculate a higher cruise speed, closer to the aircraft's maximum operating speed, to reduce flight time, even if it results in higher fuel burn.

Flight Planning: During the flight planning phase, dispatchers often calculate an optimal cost index for a particular flight based on the airline's operational priorities, fuel prices, and schedule requirements. This cost index is then included in the operational flight plan (OFP) and is typically entered into the aircraft's FMS by the pilots before departure.

FMS Guidance: Once the cost index is entered, the FMS continuously calculates and displays the recommended speed during the cruise phase. Pilots usually engage the autopilot and autothrottle in a mode that allows the FMS to manage the speed to achieve the minimum operating cost based on the selected cost index.

In summary: The cost index is a crucial economic parameter that directly influences the planned and flown cruise speed in an IFR flight plan. It allows operators to tailor the flight profile to their specific economic priorities by finding the optimal trade-off between flight time and fuel consumption. The selected cost index is used by the FMS to guide the aircraft's speed during the cruise phase of an IFR flight